One of the major problems with co-axial motor/prop configurations its loss of efficiency. Everyone pretty much agrees on about a 20% loss of efficiency with a co-axial setup. Also, there’s a slight increase in noise but that not really the tipping factor. I would be fun to experiment with some setups in the future be it’s just not ideal. Thanks for sharing.

The torque on the aircraft from a pair of contra-rotating propellers effectively cancels out.
Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers.
However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB. Most of this extra noise can be found in the higher frequencies. These substantial noise problems limit commercial applications. One possibility is to enclose th...

But searching/reading more, I think in rcgoups formus, cannot find links now, I have found that it is difficult to implement it right, to achieve extra efficiency.

My thoughts is to stick with current design and release it ASAP After that go to experiment more

Yes theoretically they can be in very very specific environments (high speed, and within very similar mach number) but for our situation, there would be at least an efficiency decrease of 15% if not more.
There is reason you don’t see the large drone and airplane manufactures making aircraft with coaxial props. You would think that with the 6-16% decrease in fuel consumption there would be a large demand. The vast majority of the time it is less efficient.

I also looked into this one with a view to e-PPG application. One of my multirotors (drones) has twin motors (counter-rotating) at the end of each arm so it drove me to set up a crude thrust jig to investigate it’s efficiency. The best number I got was an efficiency loss of 10%. This was good enough for me to keep drawing out a design for PPG. Roadblock ended up being mechanical / structural complexity of the unit. Also blocked airflow by pilots body unless going for big ass props. For me, the advantages of counter-rotating didn’t justify the challenges of implementing the design
I like the ‘strap a big drone to your back’ idea a lot better for the following reasons:
1 - smaller, cheaper motors / ESCs - same goes for props
2 - Cleaner, more simplified design (for the inevitable crash, rebuild)
3 - More unobstructed prop in clean air